Cylindrical radiant energy collectors are trough-like structures which concentrate energy incident on the entrance aperture within a certain angular acceptance onto a smaller area at the exit aperture. The structure usually includes opposed reflective walls which direct energy incident thereon within the acceptance angle, out the exit aperture or onto an energy absorber at the exit aperture. Such a collector is described in a prior U.S. application for a RADIANT ENERGY COLLECTOR, Ser. No. 492,074, filed July 25, 1974, and in a publication, Solar Energy, Vol. 16, No. 2, pages 89-95, (1974). Such collectors may be arranged in a planar array as shown in FIG. 10 of the Ser. No. 492,074 application. Since the reflective surfaces, no matter how carefully they are made, are not perfect mirrors, a portion of the energy incident on the reflective side walls is absorbed into the wall, either into the substrate which may support the reflective surface of the wall or into the reflecting material itself. For example, a typical aluminum-faced mirror would reflect, on the average, 80-85 percent of the incident radiation, the remaining energy being absorbed in the form of heat and the temperature of the reflective wall will rise. This absorbed heat is usually dissipated by radiation and natural convection and is thereby lost to the ambient environment. This loss lowers the thermal efficiency of the collector.
Further, on hazy or cloudy days, a greater portion of the energy incident on the side walls falls outside the acceptance angle of the collector. Such radiation undergoes a multiple reflection between the opposed reflective walls before being directed out the entrance aperture. This multireflection increases the temperature of the reflective walls, but this energy is lost through radiation and convection. Present means for reducing the amount of heat absorbed by the walls involves improving the quality of the mirrors to lessen the amount of energy absorbed. However, the added expense for improved mirrors negates the improved collector efficiency.
It is therefore an object of this invention to improve the thermal efficiency of cylindrical energy collectors.
Another object of this invention is to provide a means for removing heat absorbed by the reflective walls of the cylindrical collector.
Another object of this invention is to reduce the quality of mirrors required for cylindrical energy collectors.